Geomorphology

P-waves (Primary/Compressional waves) travel through solids, liquids, and gases — all states of matter. S-waves (Secondary/Shear waves) travel through solids only; they cannot pass through Earth's liquid outer core, creating an S-wave shadow zone on the far side. This shadow zone is the primary evidence that Earth's outer core is liquid. L-waves travel only on the surface. P-waves arrive first at seismographs — hence 'Primary'.

The Richter Scale measures magnitude — the total energy released by an earthquake, recorded by a seismograph. It is a logarithmic scale: each whole number represents 10× more ground motion and ~31.6× more energy released. The Mercalli Scale measures intensity — the degree of shaking and damage felt at specific locations, which varies with distance from the epicentre. Mercalli is subjective (observed effects); Richter is objective (instrument-measured).

The Himalayas formed when the Indian Plate collided with the Eurasian Plate (continent-continent convergent boundary). Since both carry low-density continental crust, neither subducts — the crust buckles upward into fold mountains. The Himalayas are still rising (~5 mm/year) as the collision continues. Compare: Andes (oceanic-continental convergent = subduction); East African Rift (divergent); San Andreas Fault (transform).

Marble is a metamorphic rock formed when limestone is subjected to intense heat and pressure underground. The calcite in limestone recrystallises into larger crystalline grains. The Taj Mahal is made of marble. Key metamorphic pairs: Limestone→Marble; Shale→Slate; Sandstone→Quartzite; Coal→Graphite→Diamond. Granite is igneous (not metamorphic). Shale becomes Slate (not Marble). These rock transformation pairs are directly tested in NDA.

The Mariana Trench is in the Pacific Ocean (near the Mariana Islands), reaching ~11,034 m at Challenger Deep. It formed where the Pacific Plate subducts under the Mariana Plate (oceanic-oceanic convergent boundary). NDA sometimes asks deepest by ocean: Pacific = Mariana Trench; Atlantic = Puerto Rico Trench; Indian Ocean = Java/Sunda Trench. The Pacific Ocean is both the largest and deepest ocean.

The East African Rift Valley is a graben — a down-dropped block between parallel normal faults, formed by tensional (divergent) forces pulling the continent apart. It is the early stage of continental splitting. The Rift Valley contains great lakes (Tanganyika, Malawi, Turkana). Contrast: Horst = uplifted block between faults (e.g., Black Forest). Fold mountains = compressional forces (Himalayas). Rift valley formation is a classic NDA distinction.

Sedimentary rocks form from compressed layers of sediment (sand, silt, organic material) deposited in water or on land. Dead organisms can be trapped and preserved as fossils during this layering process. Igneous rocks form from molten magma — extreme heat destroys any organic material. Metamorphic rocks form under heat and pressure — any pre-existing fossils are destroyed in the process. Sedimentary rocks = fossils = NDA standard answer.

Shield volcanoes have broad, gently sloping sides formed by thin, low-viscosity basaltic lava flowing long distances before solidifying. They are non-explosive. Examples: Mauna Loa (Hawaii), and all major volcanoes on Mars (Olympus Mons). Stratovolcanoes (composite) have steep sides and explosive eruptions (Mt Fuji, Mt Vesuvius). Caldera = collapsed magma chamber forming a depression. This distinction is directly tested in NDA.

Earth's outer core is composed of liquid iron-nickel. The convection currents of this liquid metal, combined with Earth's rotation, generate Earth's magnetic field through the dynamo effect. Earth's magnetic field deflects the solar wind, protecting life from harmful solar radiation. Without the magnetic field, Earth's atmosphere would be gradually stripped away (as happened to Mars, which lost its magnetic field). The inner core is solid iron-nickel.

Stalactites hang downward from cave ceilings; Stalagmites grow upward from cave floors. Both form by deposition of calcium carbonate from dripping water in limestone (karst) caves. Memory trick: stalacTITE holds TIGHT to the ceiling; stalagMITE MIGHT reach the ceiling someday. Karst landscapes also feature sinkholes (dolines), disappearing rivers, and limestone pavements — all formed by chemical weathering of limestone.

The Deccan Plateau was formed by massive volcanic eruptions ~66 million years ago (coinciding with the K-Pg mass extinction event) — lava flooded the region creating vast basaltic layers called the Deccan Traps. The weathering of this basalt over millions of years produced the Deccan's black cotton soil (Regur). This volcanic origin makes the Deccan Plateau different from fold mountains (like the Himalayas) or erosional plateaus.

Soil creep is the extremely slow (mm/year) downslope movement of soil and regolith under the influence of gravity — too slow to be directly observed but cumulatively significant over decades. It is a type of mass movement. Other mass movements by speed: Earthflow (moderate), Mudflow/Lahar (fast), Landslide/Rockfall (very fast). Soil creep is identified by tilted fence posts, curved tree trunks, and stepped hillsides (terracettes). It is the most common form of mass wasting worldwide.

The Mohorovičić Discontinuity (Moho), named after Croatian seismologist Andrija Mohorovičić, marks the boundary between Earth's crust and mantle. It is identified by a sudden increase in seismic wave velocity. Other boundaries: Gutenberg Discontinuity = mantle-outer core boundary; Lehmann Discontinuity = outer core-inner core boundary. The Moho lies ~35 km deep under continents and ~5–10 km under oceans. NDA tests all three discontinuity names.

Coal is an organic sedimentary rock — formed from the compressed remains of ancient plant matter buried under sediment over millions of years. Under increasing pressure, peat becomes lignite, then bituminous coal, then anthracite (the highest grade). Coal contains no fossils (plant tissue is transformed) but represents biological material. When subjected to extreme metamorphism, coal can become graphite or ultimately diamond. Sedimentary classification is the direct NDA answer.

The Ring of Fire is a horseshoe-shaped zone of intense volcanic and seismic activity surrounding the Pacific Ocean. It contains ~90% of the world's earthquakes and ~75% of active volcanoes. Countries on the Ring of Fire: Japan, Philippines, Indonesia, New Zealand, Chile, Peru, USA (West Coast). Major earthquakes (2004 Sumatra, 2011 Japan) occurred on the Ring of Fire. The Pacific's many subduction zones on its margins create this concentration of tectonic activity.

Mid-ocean ridges form at divergent boundaries where two plates move apart — magma rises to fill the gap, creating new oceanic crust (seafloor spreading). The Mid-Atlantic Ridge is the world's longest mountain chain (~65,000 km). Iceland sits directly on the Mid-Atlantic Ridge. Convergent boundaries create trenches and fold mountains. Transform boundaries create lateral fault systems (e.g., San Andreas Fault). This plate boundary-landform pairing is essential NDA knowledge.

The Sahara is a hot trade wind desert (also called subtropical desert or tropical desert), formed under the subtropical high pressure belt (~30°N) where dry, sinking air dominates and trade winds carry moisture away. This is the mechanism behind most of the world's great deserts. Cold deserts (Gobi, Atacama coast) form by different mechanisms. The Sahara (~9.2 million km²) is the world's largest hot desert. Note: Antarctica is the world's largest cold desert.

When wave erosion carves through a headland, it creates: Cave → Arch → Stack → Stump. A sea arch eventually collapses, leaving an isolated column of rock called a sea stack. Over time, the stack erodes further to form a sea stump (a flat-topped remnant at or below water level). The wave-cut platform is the flat rocky shore revealed at low tide after the cliff retreats. This coastal erosion sequence is a classic NDA geography question.

Loess is fine-grained, wind-blown (aeolian) sediment — primarily silt particles transported by wind from deserts and glacial outwash plains and deposited downwind. Major loess deposits: China (Loess Plateau — world's largest loess deposit), Central Europe (Rhine-Danube region), Mississippi Valley. Loess forms extremely fertile agricultural soils. China's Yellow River (Huang He) gets its colour from loess eroded from the Loess Plateau. Loess soils are highly erodible and prone to gully formation.

The focus (or hypocentre) = the actual point inside Earth where the earthquake originates. The epicentre = the point on Earth's surface directly above the focus. The epicentre experiences maximum intensity of shaking. Seismic waves radiate outward from the focus. The depth of the focus determines whether an earthquake is 'shallow' (<70 km, most destructive), 'intermediate' (70–300 km), or 'deep' (>300 km). NDA tests both focus and epicentre definitions.